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HKS Sequential Bypass Valve - Report

...That 300USD toy is PLASTIC for one, folks.  Read on:

I got to spend 4 hours with a car last night trouble shooting and testing a
HKS valve installed in an audi turbo, as well as, several conversations with
HKS technical department (good helpful folks I might add).  Again, it seems
that the legend of this puppy might be a bit overated.  
My report follows for the archives:

A brand new, HKS Sequential Bypass Valve -  Discount Cost for this particular
one is 260.00USD including steel mounting flange.  The car:  Highly modded RS2
turbo equipped 5ktq.  This valve is ONLY available as an atmospheric dump
valve, that is to say, there are no accessories or adapters available to
operate this valve closed loop (back into intake side of turbo - see
archives).  Upper chamber has boost/vacuum port with diaphram (this is exactly
like your WG folks), lower upper chamber has boost.  Boost Pressure 90 degrees
to plug operation

This latest toy from HKS is a "high temp plastic" (HKS tech quote) that is
chromed.  Has an aluminum cone piston primary relief valve that measures 1/2"
in diameter.  The secondary relief valve is a rubber plug type (like the stock
bosch, but mucho thicker) that measures 1.40" in diameter.  The primary cone
sits in a basket that is attached to the secondary plug.  After approx 1/8" of
travel the primary valve hits the basket, pulling the secondary valve open,
hence sequential.  The object is that on lighter vacuum/pressure, the smaller
valve opens (like with a shift), then on a decel mode, the secondary valve
opens, dumping more pressure and flow (see archives) out of the valve,
relieving the pre t-body pressure spike created when closing the throttle
plate on boost.  There is a spring (this operates just like a WG folks) on the
top of the diaphram in the upper chamber with manifold port.  There is a
spring preload on that diaphram as well with a locking nut (externally adj).
Pressure on the bottom of the diaphram from boost, as well as vacuum above the
diaphram from manifold vacuum opens valve.

I tested the valve with 15 to 25in of vacuum, opening neither the primary (?)
or the secondary valves.  Disassembling the valve completely and my first tech
call revealed that pressure under the upper chamber diaphram has to be present
for the manifold vacuum to open the valve.  Given the design of the shaft, it
doesn't appear to need much, but some.  I understand the operation of the
secondary valve needing lower pressure, but was surprised that the primary
wouldn't open with just vacuum.  The depth of the primary travel  is 1/8"
before secondary operation.  How much flow can a .5 X 1/8" bore valve really
help things?  I get ahead.

The valve operates smoothly, tho almost too much so, with this install, the
turbo whine (vs stall) was the indicator that the valve was working more than
the expected "valvefarter" noises associcated with bypass valves.  The install
and removal is easy with supplied circlip pliers and reusable oring gasket.
The spring preload appears to be a good mod, tho HKS suggested NO preload, and
given the high vacuum required for movement, I would concur.  The diaphram is
NOT as thick as some other valves available, but is thicker than the stock
bosch one, and given the size of the 2 vacuum/boost feeds, I would say neither
side sees much boost stress.   

A VERY expensive toy to be plastic.  HKS assured me that an aluminum one,
albiet with a slightly modified design will hit mid 1998 intro.  The 1.40"
secondary plug seems a bit small to me, tho exactly 40% larger than the stock
bosch unit.  Timed 0-60 runs of 6.39secs (44F ambient @ sealevel, 580lb above
3350 curb wt, flat road measured 5 times, rejected 3 worst, g-tech pro,
calibrated = 1.00g at vertical position, 3krpm launch sliding clutch, no
6000rpm dumps allowed folks).  No valid time difference connected or
disconnected.   This is probably do to the fact that the throttle is not
completely shut during runs, so valve not effective to time.  

Given the price, I would say 2 valves might be better than one.  The bosch
valves have many numbers that supercede, each "addressing" the hi boost
blowout failures of the plug.  The latest revision has a medal plug base and
it's part number is 034 145 710 C.  I have ordered one for testing and will
post the results.  Also available, aluminum from TAP (1in direct replacement
with really beefy diaphram, and serviceable unit, different springs
available), my personal recommendation @ 150.00 (it's RS2 anodized blue)  Also
available, a plethora of non closed loop bypass valves from TAP and others.  

Heat on the HKS.  Given the location of the IC to T-body hose, I just might
Ceramic coat some hot surrounding parts.  "No reports" of heat failures (says
HKS), but hey, plastic is... plastic  

Open loop design.  Problems here, closed loop metered air (see archives)
concerns about rich fuel spikes (O2 failure, Cat failure), and this is NOT an
emissions legal device.  Dumping raw intake tract oil to atmosphere would make
an EPA inspector cringe

Small improvement in valve area.  Not sure WHAT I was expecting, but easily,
those of you running two 1in valves at the same price have at least 25% more
flow and pressure thru the valves than the HKS equipped folks.  

Given the very small primary piston travel, I see this HKS unit as a good idea
for a small turbo.  The larger the turbo, the less benefits from the
"sequential" modus.   

Price:  This thing is bux, and B4B, another expensive mod, being a plastics
guy,  I am just surprised that some sort of medal wasn't used.  

A neat toy, hard to justify.  Reapplication of boost post "coast" noticeable
improvement, 0-60 didn't show it.


Scott Justusson
'87 5ktqwRS2
'86 5ktqw
'84 Urq